1. Field of the Invention
The present invention relates to a toner binder, a toner, an electrophotographinc printing method and an apparatus therefor. More specifically, the present invention relates to an improvement in a toner binder for flash fixing which is superior in flash fixability, void resistance and environmental electrostatic charging stability and, especially, which does not generate an unpleasant odor during flash fixing.
2. Description of the Invention
Electrophotography has been conventionally known to use a system such as that described in U.S. Pat. No. 2,297,691, where a photoconductive insulator (e.g., a photoconductive drum) is commonly used, a uniform electrostatic charge is applied to the photoconductive insulator by, for example, a corona discharge, a light image is irradiated on the photoconductive insulator by various means to form an electrostatic latent image, the latent image is developed and visualized using fine particles called toner and after the toner image is transferred, if desired, to paper or the like, the toner image is fixed onto the recording medium such as paper by means of pressing, heating, exposure to solvent vapor or irradiation by light to obtain a printed matter.
The toner used for developing the electrostatic latent image is conventionally produced by dispersing a coloring agent such as carbon black in a binder resin comprising a natural or synthetic polymer material and finely granulating the obtained dispersion into particles of approximately 5 to 20 .mu.m in size. The toner may be used in the development of an electrostatic latent image as the toner by itself or as a mixture with a carrier such as iron powder or glass beads. In the case where iron powder or other ferromagnetic powder is used as the carrier, the development is conducted in such a manner that the developing agent consisting of a toner and a carrier is mixed and stirred in a developing apparatus to be charged with frictional electrification, a magnet roll in the developing apparatus is rotated to form a magnetic brush, the formed magnetic brush is transported to the electrostatic latent image portion on the photoconductive sensitized material as the magnet roll rotates and only the charged toner is attached onto the latent image due to the electrical attraction force. After the development, the developing agent reduced in the toner density is replenished by new toner, to maintain the toner density constant, and can be repeatedly used.
On the other hand, the toner powder image formed on the photosensitive drum is transferred onto a recording medium (e.g., paper) by corona transfer or roller transfer. The toner powder image transferred to the recording medium is adhered to the paper in the state of powder forming an image, where if it is rubbed by, for example, a finger, the powder image is damaged. In order to fix the toner powder image on the recording medium, the powder image must be melted to fix it to the recording medium and one of the various methods described above is used therefor. Among these methods, flash fixing as a representative example of photofixing is conducted by a flash of light from discharge tube such as a xenon flash lamp and is characterized as follows:
1) due to non-contact fixing, the resolution of the developed image is not deteriorated, PA1 2) the stand-by time after a power source is turned on is not required and a quick start can be realized, PA1 3) even when the recording paper is jammed in the fixing device due to a system failure, ignition is not generated, and PA1 4) fixing is possible irrespective of the material or thickness of a recording paper such as pasted paper, preprint paper or other papers different in thickness.
The toner is fixed to the recording paper by flash fixing through the following procedure. As described above, the toner image adheres to the recording paper in the state of powder forming an image when it is transferred to the recording paper but the powder image is readily damaged by rubbing by, for example, a finger. When a flash of light from a discharge tube such as a xenon flash lamp is irradiated thereon, the toner absorbs the energy of the flash of light to raise the temperature of the toner and thereby the toner is softened and melted to firmly adhere to the recording paper. After the flash of light finishes, the temperature falls to solidify the image and a fixed image is obtained, thus accomplishing the fixing, and the fixed image firmly adhered to the recording paper cannot be damaged even if it is rubbed by, for example, a finger.
Referring to FIGS. 1A to 1C, it is important in flash fixing that the toner 1 is melted and firmly adheres to the recording paper 2 and therefore, the toner must be melted by absorbing from the flash of light sufficient light energy which must include the energy used for melting the toner and the heat energy diffusing outside the toner not contributing to the increase of temperature. Accordingly, if the light energy given is insufficient, the toner cannot be melted sufficiently and as a result, the fixing is not satisfactory. On the other hand, if the light energy 3 is too high, the viscoelasticity of toner 1 is abruptly lowered (FIG. 1B). At this time, if the surface tension of the toner 1 overpowers the viscoelasticity, the toner on the printing portion aggregates and the toner moves to bring about a blank phenomenon called a void 5 on the fixed image to thereby cause a reduction in the image density (FIG. 1C). Accordingly, the toner for flash fixing must not generate voids 5 by moving the toner and in this concern, the use of a binder resin having a high viscosity when melted is needed or the use of a binder having a low surface tension when melted is demanded.
Further, as one of the important characteristics of the toner for flash fixing, the binder resin constituting the toner must melt rapidly in the course of fixing to paper and the like and after it is cooled and solidified, should show good fixation. In order to obtain such a toner property, a polymer having a low molecular weight and a low melt viscosity, generally called an oligomer (for example, having a number average molecular weight Mn of less than 1500 and a weight average molecular weight Mw of 10,000 or less) is widely used. However, since the oligomer has a low molecular weight, the glass transition point is low and therefore, there arise problems such that (1) the storage stability of toner is low, (2) blocking of toners readily occurs in a developing machine, (3) the toner is readily fused in a developing machine or a fused product (such as coarse toner) is easily formed and (4) the toner properties are liable to alter due to a change in the operating environment of the apparatus (e.g., temperature or humidity). The reason why many problems described above are caused when an oligomer having a low molecular weight is used is that if the molecular weight is made low to give the binder a low melting point, the glass transition point is also lowered, often to the room temperature level.
Therefore, in order to provide excellent flash fixability and solve the problems described above, it is necessary to optimize the melting point and the glass transition point of a binder used for the toner and it is also necessary to develop a toner comprising a binder having a low melting point and a high glass transition point.
As to the properties which the toner for flash fixing is required to have, the binder resin constituting the toner should not generate an unpleasant odor when a toner is heated to a high temperature during flash fixing. The generation of an unpleasant odor becomes a problem when the printing speed per hour is increased in flash fixing and this is because the quantity of toner melted per hour increases. For example, in the case where a printing speed is 300 mm/sec and the printing pattern is general pattern (i.e., the area ratio of the area attached by toner/the area not attached by toner is about 0.05), the quantity of toner melted per one hour is about 100 g. In an apparatus for high-speed printing at such a speed, an unpleasant odor is easily generated and becomes a serious problem in a high speed apparatus.
A technique to prevent generation of an unpleasant odor on fixing is proposed in Japanese Unexamined Patent Publication (Kokai) No. 63-193155. Japanese Unexamined Patent Publication (Kokai) No. 63193155 discloses that the generation of an unpleasant odor on fixing is ascribable to an acid monomer among residual monomers (non-reacted monomers) contained in the toner binder. It is an effective method to use a binder toner containing little of components generated by vaporization or sublimation of the above-mentioned nonreacted monomers during fixing.
However, the toner temperature in flash fixing is said to reach 200.degree. C. or higher at the surface of the toner, as described, for example, in Japanese Unexamined Patent Publication (kokai) No. 4-56869, and under such high temperatures, an ordinary toner binder is likely to undergo thermal decomposition to generate thermally decomposed gas components which give rise to an unpleasant odor. Since the toner temperature in flash fixing reaches the thermal decomposition temperature of the binder as described above, the generation of thermally decomposed gas components cannot be avoided. Accordingly, even if the method described in Japanese Unexamined Patent Publication (kokai) No. 63-193155 is used, the fixing odor cannot be nullified and as a means to solve the problem of a fixing odor in flash fixing, methods using binders which generate a small amount of thermally decomposed gas components even when the toners are melted at high temperatures as described above or using binders which generate thermally decomposed gas components free of odor should be employed.
The object of the present invention is to provide a binder for use in a toner for flash fixing which is superior in flash fixability, void resistance, electrostatic charging stability and storage stability and, especially, which does not generate an unpleasant odor on flash fixing, a toner using the binder, an electrophotographing method using the toner and an apparatus therefor.